1. Field of the Invention
The invention relates to a method for natural suppression of sprouting and improvement of internal processing quality of stored potatoes.
2. Description of the Prior Art
Typically, tubers are harvested, allowed to suberize (i.e., allow the "skin" or periderm layer to form over wound areas) at warm temperatures for about 10 days, then gradually cooled down to the storage temperature of about 10.degree. C. For the first 1-2 months after harvest, the tubers remain dormant and exhibit little inclination to sprout. However, after this period the tubers must be chemically treated or refrigerated at very cold temperatures to prevent sprouting from occurring, such sprouting causes numerous deleterious effects to the tubers. These include a loss of fresh weight, the conversion of starch to sugars, and a decrease in the quality and appearance of tubers sold fresh. Sprouts and the surrounding tissue also contain elevated levels of toxic glycoalkaloids, which are not destroyed by cooking.
In addition to the problems encountered with sprouting, stored potatoes convert starch to reducing sugars as they age and as a result of cold temperatures and other stresses. The development of reducing sugars in the raw product is very undesirable because the sugars combine via aldol condensations with amino acids during processing to form dark melanoidin pigments. Even small accumulations of reducing sugars in the raw product result in unacceptably dark and unmarketable finished product. Currently, the only technology available to partially control the accumulation of reducing sugars in stored potatoes is to store them at intermediate temperatures (approximately 9.degree. or 10.degree. C.), to minimize cold stress, and then slowly warm them before marketing to hopefully respire surplus sugars before processing. This procedure is not always successful and it precludes storage at cold temperatures which could otherwise be employed to inhibit disease and weight loss. Presently there is no effective means to biologically control or reduce the level of reducing sugars in potatoes. The inability to control or dissipate accumulation of reducing sugars in potatoes results in over 40 million dollars in losses each year and impedes the ability to develop new markets.
Chlorpropham (CIPC; 1-methylethyl-3-chlorophenylcarbamate) is currently used to control tuber sprouting throughout the industry. Although CIPC has been used effectively, it has been on the market for over three decades and no replacements or improvements to the technology of sprout control have been made commercially available during this time. In the U.S. and around the world, there is increasing emphasis on replacing synthetic control agents (agricultural chemicals) with natural biological control mechanisms that are safe and more environmentally acceptable.
For many centuries, the Incas of South America and their descendants have buried potato tubers in pits that are layered with soil and the leaves of Muna plants that belong to the mint family Lamiaceae, and the genera Minthostachys and Satureja. This treatment prevents sprouting and excessive fresh weight loss, and repels insect pests. These Muna plants contain copious amounts of essential oils that are substantially comprised of monoterpenes. Aliaga and Feldheim [Ernahrung, 9:254-256 (1985)] and Feldheim ["Practicability and Mode of Action of Quality Storage of Potatoes After Harvest," In Report of a Lecture Given to the German Institute for Quality Research (Plant Nutrition Products), March 1985, 6 pages] reported that the oil from the Muna plants was more effective than CIPC in inhibiting sprouting, fresh weight loss, and the incidence of rotted tuber parts over a period of 120 days. The authors also reported that the main components of the oil, including the monoterpenes .alpha.- and .beta.-pinene and limonene, and the oxygenated monoterpenes pulegone and menthone/isomenthone, are effective in this regard.
Currently, several research groups in the United States and Europe are investigating alternative synthetic chemical inhibitors to tuber sprouting [Rama and Narasimham, J. Food Sci. Technol., 24:40-42 (1987)].
Vaughn et al. (U.S. Pat. No. 5,139,562) and Vaughn and Spencer (U.S. Pat. No. 5,129,951) disclosed that the oxygenated monoterpenes cineole, fenchone and menthol, as well as several aromatic aldehydes and alcohols, including thymol, hydrocinnamaldehyde, cuminaldehyde, salicylaldehyde, cinnamaldehyde, and benzaldehyde, may be advantageously used to inhibit tuber sprouting, fresh weight loss, rotting, and fungal growth.
Jasmonates are cyclopentanone compounds which are commonly present throughout the plant kingdom. The structures and bioactivities of jasmonates are thoroughly reviewed by Hamberg and Gardner (1992, Biochimica et Biophysica Acta, 1165:1-18) and Parthier (1991, Bot. Acta, 104:446-454), the contents of each of which are incorporated by reference herein. The structure of the free acid, jasmonic acid (JA), is as follows: ##STR1## Jasmonates have been described as exerting a wide range of differing effects on virtually all plants, ranging from inhibition to promotion of plant processes. As described by Parthier, the effect exhibited on the plant may even be concentration dependent, with some processes stimulated at lower concentrations but inhibited at higher concentrations. A few examples of activities influenced by jasmonates include inhibition of seed germination and seedling growth, stimulation of seed germination (at lower concentrations), promotion of seed dormancy breaking, and promotion of leaf senescence. Ryan and Farmer (International patent application WO 91/18512, published Dec. 12, 1991) disclosed treating plants with jasmonates to induce production of defense proteins. Treatment of tomatoes, tobacco, alfalfa and transgenic tobacco plants was specifically disclosed. Tazaki (Japanese kokai 2-92220 (A), published Apr. 3, 1990, patent application no. 63-242432, filed Sep. 29, 1988), Yoshihara et al. (1989, Agric. Biol. Chem., 53:2835-2837), Matsuki et al. (1992, Biosci. Biotech. Biochem., 56:1329-1330), and Koda and Okazawa (1988, Plant Cell Physiol., 29:969-974) all disclosed treating potato stem fragments with jasmonates in culture to induce tuber formation.